Process for treating cyanide solutions



e n w SOLUTIONS a. F- Q QnF ii Ta lin l' 110 N on'l ees, v England, 'assigiiorsf Imperialf H l'lndustri' jLimited,-a corporation-of" ,A-pplication May 11,1933, Serial No.6'10,590

In Great Britain May-24,1932 I 4 Claims. ((71. 75-485) 1 invention relates to an improved method of treating cyanide'solutions fortherecovery of cyanide: and/or metal values therefrom, and in particular-to the:treatment iorcyanide solutions containingprecioii's metals:- 'Iheextractiofi of gold fron'i--'- ores by means of a dilute; alkali 'metalor-alkaline earth metal cyanide solution and the subsequent precipitation of the gold'by =means iofzinc' dust or shavings followedbyre-use ofzthesolution for the extractionaof more gold; is axwell' known process. It is alsowell-known that the gold may be recovered from the "cyanide solution by: electrodeposition.

1: Cyanide solutions which. have been used for the extraction-10f:an ore="may,;-contain, in addition to preciousmeta-l cyanide Tcompounds, appreciable amounts or complex- 1 cyanides= .of which base metalsxare constituents; and .;there may also be thiocyanatesin- 501111710115 For. example, this may occur when 1 the sore treatedzcontains iron sulphides. These complex basemetal cyanides and thiocyanates; result ,-;:from the action of the cyanide solution on:- the;, ore and .their amount and hara t s nea rmfthetyp i e. but in any case their formation causes a conversion of the $1 i an de inia sub tan without .1 pr am le. olvent act n 1 preciou metal values in the ore. is obviously a -disadvantage when the cyanide soluti r'iis re-us'ed foil further extractiona'fterthe precipitation, operation. In addison to the] i reased cyganide, onsumption there maybe a fi h tjdisadv tae wi o h cultiesflarisin ftliefprcipit tionot the gold. )F'njf ap-meat. ee v nd that thereis 3' consi erable plying the cyanide process tof certain g be ,ing ores which contain copperifi thef extraction oflsuch ores the cyanide solution dissolves copper addition to the goldjandgas a. result, difllculties arise in the e e r" b h. ,i hliw hgz jt or by el'ectr'odepos'itiori'; In the'c seotprecipitaf ti n b zi c, atin e 9 9 s termed he zinc whic "gradually inh' tliei "ecipitation of complete-recovery imam a Jeed associated with the zinc,

" eni hd i ei' l l. the j recirculation of i th a j'pe s t e'bui qfi f .j 7 1th? $91961 im ce in 1 11. a emam d 1 ypurging peruqn' oi "ecopper' bearing yields off gold but or by increasing the s; ide j d eeiitritidng Both sary. ,In addition to the above there istthe further disadvantage that the gold recovered from copper bearing solutionsis contaminated with copper and the subsequent separation which is necessary is an additional expense. Difliculties also occur when other base metals are present, although the difliculties are not so great. I The foregoing paragraphs apply equally to the recovery 01 silver as well as to the recovery of gold. v

According tothe-presentinvention we treat a cyanide solution, containing precious metals, with a cuprous salt in sufiicient amount to precipitate all the cyanide together with all the precious metal values, the solution for this purpose being maintained substantially neutral or slightly acid. According to a further feature of the invention we recover cyanide values and precious metal andcopper values from the separated precipitate, which consists of cuprous cyanide together with cyanide compounds of precious and any base metals, by treatment with a chloride 01' an alkali or alkaline earth metal, and a suit.- able acid such as sulphuric acid. By this treatment cyanide values are converted to hydrocyanic acid which may be recovered in any suitable manner; the precious metals and any copper thiocyanate remainas an insolubleresidue, and the remaining copper and base metals are dis-'- solivednz i I The invention will be better understood by reference tothe accompanying drawing, which is a diagrammatic flow sheet. The process described, with reference to this flow'sheet, is appliedto a solution obtained in the cyanideprocess for the extraction of gold ore, and'which contains free is e'' d t draw at on muchgreatef quantity thanis otherwisenece'scuprous salt solution may be prepared by the reduction of copper sulphate by means of sulphur dioxide and metallic copper in presence of to be easily detectable in the filtered solution;

(Copper is detected in a sample of the solution after oxidation by nitric acid. by the well known blue colour produced on adding ammonia.) The point at which a just detectable amount of copper is present in solution is that at which the minimum copper addition has beenmade. It is advantageous from the point of view of gold precipitation to add the cuprous salt solutionin considerable excess above this minimum. An excess up to 25% of over the minimum amount may be added, but 15% will usually be sumcient.

After precipitation is complete the reaction mixture is passed by pipe 5 to a filter 6. 'a filter' press of any usual type beingsuitable. The liquor leaving the filter by pipe '1 contains no valuable constituents, with the exception of any ex-" cess cuprous salt which may have been added in the previous operation, and may be run to waste. or if, desired. it may be treated forthe recovery of copper by passing it to the tank 24, the function of which is described later.

The solids separated by filtration and removed at 8 consist of complex cyanides of copper and iron. copper thiocyanate. and gold and silver salts. These solids are next heated, preferably by live steam, with sulphuric acid and sodium chloride in a still 9 of suitable resistant material, so as to liberate hydrocyanic acid and to dissolve the heavy metal constituents. The amount of sulphuric acid must be slightly in excess of that equivalent to the recoverable cyanide combined with copper and iron, while the amount of sodium chloride used must be that required to form cuprous chloride from the copper present, to-

the steam added for-heating; the final quantity of water present should be sufiicient to dissolve the soluble base metal compounds formed, but is preferably maintained as near this lower limit as possible in order that the evolution of hydrocyanic acid may be as complete as possible. We

have found that if these conditions areobserved. over 90% of the, hydrocyanic acid can be recovered. In Y The evolved vapours. consisting of hydrocyanic acid and steam, are passed by pipe 10 through condenser 11 to an absorption vessel 12 where they are treated with milk of lime, thereby formfor use in the extraction of further quantities of ore. Additional cyanide is added to the solu; Y tion withdrawn at 13 to make up for the consumption of cyanide due to-formation of thiocyanates, mechanical losses and incomplete evolution of .lution mentioned in connection with the said acidifying and then precipitating cuprous cyanideing a calcium cyanide solution which is suitable washed with a slightly acid brine to remove adherent cuprous chloride. The solids in the filter cake consist essentially of copper thiocyanate, gold cyanide, and silver, probably in the form of chloride. These solids are removed from the 5 filterpress 14 and 15 andpass to a roasting furnace 16 where the copper thiocyanate is converted into oxide. The solid product is then passed to a vessel 1'! in which it'is treated with dilute sulphuric acid. The copper oxide is disl6 solved-and the silver and gold remain in the form of a sludge. The mixture is passed,by pipe. 18 to a filter 19, and the solid cake containing gold and silver is removed at 20. It is then worked up in the usual manner into bullion.

- 15 The liquor from the filter 19 contains copper sulphate and is conveniently joined with the liquor proceeding from the filter 14 andthe combined liquors are passed by pipe 21 to a vessel 22, in which they are subjected in a slightly warmed condition to the action of metallic copper and if desired, of sulphur dioxide in-order to reduce any cupric. copper to the cuprous form. It may be found, due to mechanical losses, that there is insufilcient copper in the liquor fed in by 23 pipe 21 to satisfy the requirements of the precipitation carried out in vessel 3. If this is the case then additional copper may be introduced as metal in the reducing process carried out in vessel 22. On the other hand, if there is an appreciable amount of copper present in the initial liquor from 22is more than sufiicient to'suifice for the precipitation in the vessel 3. In this case a portion of the reduced liquor may be passed by pipe 23 to a vessel 24 in which metallic copper is precipitated by means of scrap iron. The recovered copper may be utilized for the reduction process and there may even be a surplus of .the metal in' the event of an appreciable quantity of copper being. dissolved from the ore by the cyanide solution; a

The freshly reduced solution of cuprous salt" from the vessel 22 will be strongly acid and it will also contain a considerable quantity of iron salts. In certain circumstances it may be advantageous to remove substantially all the acid and even'the iron from this liquor. .If this is found to be the case. neutralization can be eifected'by the addition of calcium carbonate and heating with an excess of this agent will also eifectively precipitate the iron, which can then be removed by filtration.

The inven ion is not restricted to the precise form of process described above .with reference to the drawing, nor is it restricted to the treatment of the particular type of initial cyanide soprocess.

We are aware that it has been proposed to re cover gold-from gold bearing cyanide solutions by along with aurous cyanide by addition of cuprous chloride. In this proposal it was stated that only a very small quantity of cuprous chloride sufilces to procure a complete separation of the gold and that after separation of the gold-cop- 7 per precipitate the. liquor contains all the cyanide as free hydrocyanic acid, less the small quantity combined in the precipitate. Neutralimflon 0! the free hydrocyanic acid with alkali gim the solution of cyanide for the next' extraction. 1h 15 practical working this process possesses the marked disadvantage that during practically the whole operation free hydrocyanic acid is present in the acid solution and may easily be evolved and lost. It has also been found that under the conditions specified there is only a very incom plete separation of the gold from solutions such as are encountered in practice.

we claim:

1. A process for the recovery of cyanide and metal values from cyanide solutions which have been used for the extraction of precious metals, the said solutions containing free cyanide, alkali,

k precious and base metals in the form of complex with acid to dissolve out the copper and leave the precious metals as an insoluble residue, and

" utilizing the dissolved copper for precipitating the cyanide and precious metal values from a further quantity of the initial cyanide solution.

2. A process for the recovery of cyanide and metal values from cyanide solutions which have been used for the extraction of precious metals, the said solutions containing free cyanide, alkali, and precious and base metals in the form of complex cyanides, which comprises treating the solution with an aqueous solution of alkali metal chloride containing dissolved cuprous chloride,

said solution of cuprous chloride being added in sufficient amount to precipitate all the cyanide together with all the precious metal values, separating the precipitate consistingessentially of cuprous cyanide together with cyanide compounds of precious and any other base metals; treating the said precipitate with an alkalimetal chloride together with sulphuric acid in the presence of water whereby the cyanide values are converted to hydrocyanic acid and the copper values previously combined as cyanide are dissolved as cuprous chloride in the aqueous solution of alkali metal chloride, driving off said hydrocyanic acid and recovering cyanide values corresponding thereto, separating from the insoluble' residue containing the precious metals the aqueous solution containing cuprous chloride and alkali metal chloride and utilizing said solution for precipitating the cyanide and precious metal values from a further quantity of the initial cyanide solution.

3. A process as set forth in claim 2, in which the aqueous solution containing cuprous chloride and alkali metal chloride which has been separated from the insoluble residue containing the precious metals after the hydrocyanic acid has been driven off, is treated for the reduction to a cuprous form of any cupric copper ltmay contain prior to use in the precipitation of further cyanide and precious metal values.

4. A process as set forth in claim 2, in which the aqueous solution containing cuprous chloride and alkali metal chloride which has been separated from the insoluble residue containing the precious metals after the hydrocyanic acid has been driven off, is treated for the removal of iron,

priorto using it for the precipitation of further cyanide and precious metal values.

THOMAS EWAN. REGINALD JOHN LEMMON. 

